1. Field Of the Invention
The present invention relates to the field of high speed digital circuits, in particular, high speed digital circuits based on CMOS technology. More specifically, the present invention relates to a method and apparatus for testing frequency symmetry of digital signals propagating in high speed digital circuits of a digital system, such as a data instrumentation system.
2. Background
In a number of digital applications, such as data instrumentation, it is often necessary to determine whether two digital signals are symmetric to each other in frequency. A particular example is when digital clock skew is digitally compensated for a high speed digital circuit by reconstructing the digital clock, as the digital clock passes from one high speed digital circuit to another. More specifically, when the reconstructed digital clock is generated by a flip flop using the entering digital clock, a constant high enable and a clear signal.
Under this approach, the reconstructed digital clock is generated by clocking the constant high enable into the flip flop at the rising clock edge of a clock period. Concurrently, the constant high enable and its complement are clocked out of the flip flop. The complement output, after having been properly delayed, is used to assert an active low at the clear input of the flip flop clearing its content as well as it outputs, which in turn causes the active low to be deasserted. As the process continues, the reconstructed digital clock is generated. However, if the active low is not deasserted before the rising clock edge of the next clock period, the reconstructed digital clock will have at most half of the frequency of the entering digital clock.
Thus, it is desirable to be able to test and determine whether the entering and the reconstructed digital clocks are symmetric to each other in frequency. As will be disclosed, the present invention provides such a method and apparatus, which advantageously achieves the desirable results. As will be obvious from the descriptions to follow, the present invention has particular application to high speed data instrumentation systems.